The present invention relates to marine risers, and in particular to ccuplings for connecting marine riser section together. In particular, the marine riser may be used for drilling or for workover/completion.
Marine risers are conduits which connect an oil rig or vessel to a blow out preventer (BOP) which is stationed on or near the ocean bed. Marine risers consist of a large bore pipe or conduit. Marine risers also have additional (auxiliary) lines (such as choke, kill, and hydraulic lines) which are secured to the marine riser at regular intervals throughout its length.
Marine risers vary in size from approximately 13 xe2x85x9c inches O.D. to 24 inches O.D. and may need to be up to 10,000 feet long for deep water sites, so they are stored in sections. The marine riser sections are typically 15 to 25 metres long and are situated on the oil rig or vessel for assembly when required. Thus, when a mariner riser is to be assembled for a deep water site, up to approximately 200 joints may be needed if 15 m marine riser sections are used. It will be appreciated that such joints require to be quickly assembled and to have excellent seal integrity over a prolonged period of time to prevent either ingress of sea water or loss of drilling mud. In addition, the choke and kill lines must be 100% reliable and seal under all conditions to implement the xe2x80x9cwell controlxe2x80x9d procedure.
Existing marine risers are assembled using one of two types of couplings. The first type of coupling is the flange coupling, where each marine riser section has a flange to each end. Marine riser sections are connected by bolting the flanges together. The second type of coupling is the mechanical coupling arrangement which uses actuatable dogs in one end of a marine riser section to engage a recess in one end of a different riser section.
An example of a flange coupling arrangement is shown in FIG. 1a, and an example of a mechanical dog-type coupling arrangement is shown in FIG. 1b (which is a Cameron Iron Works Inc. riser joint assembly).
In the flange coupling arrangement of FIG. 1a, when two marine riser sections are to be joined, the corresponding flanges from each riser section are brought together, the flanges are aligned, and they are bolted together or otherwise clamped in placed. This is a time consuming process, and also the flanges required to form a secure connection are both large in size and heavy in weight.
The mechanical coupling arrangement provides for easier and faster make-up but is more complex, much more expensive to manufacture, and has a lower maximum load rating than the flange coupling arrangement.
However, these two types of coupling arrangements allow the additional (auxiliary) lines (such as the choke, kill, hydraulic, and mud booster lines) to be permanently secured parallel to the marine riser at various locations along its length, and the auxiliary lines are automatically aligned as marine riser sections are continuously added to the marine riser during assembly.
Recently, threaded connectors similar to those used in drilling tubulars have been used on production risers where provision of gas tightness is essential; however, the use of threaded connectors has been limited to risers of small diameter (O.D. 10 {fraction (1/4)} inches) to the platform.
An object of the present invention is to provide an improved riser coupling which obviates or mitigates one of the aforementioned disadvantages associated with existing couplings.
This is achieved by using couplings having screw-threads for securing marine riser sections together and by coupling the auxiliary lines to the marine riser sections at various locations along the sections to permit the lines to rotate freely relative to the marine riser sections as the sections are rotatably assembled.
Screw-thread couplings are less expensive to manufacture and provide a stronger connection than either the flange or the mechanical couplings for a given weight allowance. They also provide faster coupling and are lighter than existing couplings for a given load rating.
According to a first aspect of the present invention a marine riser comprising: a plurality of marine riser sections, each marine riser section having screw-threaded ends and coupling means for connecting said screw-threaded ends together; rotary drive coupling means for engaging with a rotary drive mechanism; each marine riser section having a plurality of spaced brackets secured to the marine riser section along its length; auxiliary line support means for receiving at least one auxiliary line, said auxiliary line support means being rotatably mounted to said spaced brackets whereby, when two marine riser sections are coupled together by rotary movement the auxiliary line support means is free to rotate relative to the riser sections to minimise rotary forces being applied to the auxiliary lines and allow vertical alignment of auxiliary lines for connection.
Preferably, an auxiliary lines coupling mechanism is provided for coupling auxiliary lines on adjacent riser sections together.
Preferably, the auxiliary line support means is a rotary carousel mechanism which comprises an annular disc with a clamp corresponding to each auxiliary line for receiving an auxiliary line. Conveniently, the clamp is large enough to accommodate the coupling mechanism for the auxiliary lines.
Conveniently, the auxiliary lines coupling mechanism is a sliding sleeve. Alternatively, all auxiliary lines may be simultaneous lifted upwards within the carousel mechanism by a hydraulic/pneumatic jacking system to provide access for the rotary drive mechanism. In this case when the auxiliary lines are subsequently lowered a threaded connection with a xe2x80x9chammer unionxe2x80x9d system, such as a FMC Dynetor or WECO coupling may be used to provide a metal-to-metal seal.
Preferably, the male and female screw-threaded ends have tapered screw-threads, conveniently, self-aligning tapered screw-threads. Alternatively, said male and female screw-threaded ends have parallel screw-threads.
Preferably, the male and female screw-threaded ends are welded to the riser sections.
Preferably, the rotary drive coupling means is a circumferential toothed rack. Alternatively, the rotary drive coupling means is a vertical keyway into which drive keys can be engaged.
Preferably, the rotary drive mechanism comprises rotary drive pinions and a drive unit. Alternatively, the rotary drive mechanism comprises a drive keys that are driven by a xe2x80x9csplit ringxe2x80x9d which, in turn, is hydraulically or pneumatically driven.
Preferably, the auxiliary line support means is mounted in the spaced brackets by hydraulic bearings. Alternatively, these bearings may be pneumatic or mechanical.
According to a second aspect of the present invention there is provided a marine riser section having screw-threaded ends and coupling means for connecting said screw-threaded ends together; rotatable drive coupling means for engaging with a rotary drive mechanism; a plurality of spaced brackets secured to the marine riser section along its length; auxiliary line support means for receiving at least one auxiliary line, said auxiliary line support means being rotatably mounted to said spaced brackets, whereby the auxiliary line support means is free to rotate relative to the riser section to minimise rotary forces being applied to the auxiliary lines and allow easy rotation of auxiliary lines to enable connection of the sliding sleeve mechanism.
Preferably, the auxiliary line support means is a rotary carousel mechanism which comprises an annular disc with a clamp corresponding to each auxiliary line for receiving an auxiliary line with a clamp in each guide for receiving an auxiliary line. Conveniently, the clamp is large enough to accommodate the auxiliary lines coupling mechanism.
According to a third aspect of the present invention there is provided a method of connecting marine riser sections together to form a marine riser, said method comprising the steps of: supporting a first riser section to display a first screw-thread end for coupling; locating a second riser section above the first riser section, where the second riser section has a second differential screw-thread end for coupling to the said first end; rotating said second riser section such that the first and second ends from a screw-thread coupling; and coupling at least one auxiliary line to the riser section and allowing the auxiliary line to rotate freely relative to the riser to facilitate vertical alignment with adjacent riser sections.
Preferably, the first riser section is supported by means of a riser spider. Conveniently, the riser spider may be achieved to support the first riser section and retracted to release the first riser section.
According to a fourth aspect of the present invention there is provided a fairing for use with a marine riser, the fairing comprising a first generally cylindrical portion for coupling to the riser and a second tapered portion coupled to the first portion, the fairing including buoyant material.
Preferably, the first and second portions are moulded from buoyant material. Alternatively, only one portion is moulded from buoyant material.
Preferably, the second portion has an insert made of buoyant material.
Preferably, the buoyant material is non-absorbent, conveniently the non-absorbent buoyant material is syntactic foam.
According to a fifth aspect of the present invention there is provided a fairing-shaped buoyancy module for use with a marine riser, said buoyancy module having carousel coupling means for coupling the buoyancy module to a carousel mechanism mounted on said riser, said buoyancy module being rotatable relative to said riser, said buoyancy module having a tapering fairing portion coupled to the carousel coupling means, the tapering portion being formed of a buoyant material and defining within the boundary of the tapering portion at least one passageway for an auxiliary line. The tapering portion can include a plurality of passageways for receiving a plurality of auxiliary lines. The buoyancy module can also include a nose portion for receiving at least one auxiliary line. Conveniently, the buoyancy module is moulded onto a carousel mechanism and is formed of syntactic foam.
According to a sixth aspect of the present invention there is provided a buoyancy module formed of a buoyant material and being moulded to have a fairing portion, said buoyancy module being adapted to be disposed on a marine riser and to rotate relative to said marine riser, said buoyancy module, when disposed on said riser, produces a minimal increase in effective riser diameter and said fairing in at least one passageway for an auxiliary line. The buoyancy module can have a bulbous nose portion defining at least one passageway for an auxiliary line.